The present invention relates to torque control of brushless DC motors.
1. Description of the Prior Art
Brushless DC motors achieve brushless commutation by the use of a resolver which is rotationally coupled to an armature shaft and a pair of resolver windings, respectively producing output signals sine .theta. and cosine .theta. wherein .theta. is the angular position of the shaft, which are used to control the windings for driving the armature. The torque produced by the armature is a function of a scaling factor applied to the sine .theta. and cosine .theta. signals which are sensed by the sensing windings.
FIG. 1 illustrates a prior art system which is sold by MPC Products Corporation of Skokie, Ill., as Part Nos. W 35 A and W 36 A. The armature 12 is coupled to a resolver 14 by the rotor shaft 16. The resolver functions in combination with a resolver excitation oscillator 18 and a pair of resolver demodulators 20 and 22 to produce output signals from the respective resolver demodulators proportional to sine .theta. and cosine .theta. wherein .theta. is the rotational position of the armature 12 and resolver 14. The torque T produced by the armature 12 is controlled by a torque command 24 which is multiplied in multipliers 26 and 28 to respectively produce control signals Tsine .theta. and Tcosine .theta.. The respective signals Tsine .theta. and Tcosine .theta. are applied to pulse width modulation amplifiers 30 and 32. In this system, the motor torque is controlled by varying the amplitude of the scaling factor for the sine and cosine voltages sensed by windings 20 and 22. The pulse width modulation amplifiers 30 and 32 have a threshold below which applied control signals Tsine .theta., Tcosine .theta. do not cause the production of a driving signal to be applied to the windings 34 and 36.
A brushless DC motor controller similar to that described above with regard to FIG. 1 has been produced which utilizes digital processing. In this system, the sensed sine and cosine signals from the resolver windings are converted to digital, multiplied by a digital torque command and used to control a digital pulse width modulation circuit. With this system, the torque is controlled by the variation of the duty cycle of the digital pulse width modulation circuit.
U.S. Pat. No. 4,520,300 discloses a brushless DC motor which multiplies the sensed signals from the resolver winding sine .theta., cosine .theta. with a torque value T to produce torque control. Driving pulses applied to the armature windings are reset when the instantaneous amplitude of a current feedback pulse overtakes the amplitude of the torque control signals Tcosine .theta. or Tsine .theta..